The invention relates to so-called panel loudspeakers operating according to the bending wave principle, in particular to positioning the drivers of panel loudspeakers.
Sound reproduction devices that operate according to the bending wave principle are known in the art. Such devices are formed essentially of a sound panel and at least one drive system, wherein oscillations are induced in the sound panel when electrical audio frequency signals are supplied to the drive system(s). According to one feature of this type of sound reproduction device, a xe2x80x9cbending wave radiationxe2x80x9d is enabled above a lower limit frequency, also referred to as critical frequency, wherein the bending waves in the plane of the respective sound panel cause the sound to be radiated in a direction that is frequency-dependent. In other words, a cross-section through a directional diagram shows a main lobe with a frequency-dependent direction. These conditions are valid for panels and absorbing panels with an infinite surface area. However, the conditions applying to multi-resonance panels (also referred to as distributed mode loudspeaker) which are the subject matter of the present application, are significantly more complex due to severe boundary reflexes. The increased complexity of multi-resonance plates is caused by a plurality of additional main lobes which are superimposed on the so-called main lobe which has a frequency-dependent direction, thereby producing a strongly fanned-out directional diagram which also has a strong frequency-dependence. Typically, the directional diagrams of the multi-resonance plates described herein are on average oriented away from the surface normal. This characteristic has the effect that the surrounding space plays a much greater role in the projection of the sound waves.
The panel of the panel loudspeaker is constructed according to a sandwich principle, in that two opposing surfaces of a very light core layer are connected, for example through an adhesive bond, by a cover layer that is thin compared to the core layer. The material used for the cover layer should have a particularly high dilatational wave velocity to enhance the sound reproduction characteristic of the panel loudspeaker. Suitable materials for the cover layers are, for example, thin metal foils or fiber-reinforced plastic foils. The core layer also has to meet certain requirements since this layer should have a very small mass density (e.g., 20 to 30 kg/m3). In addition, the core layer should be able to sustain high shear forces perpendicular to the cover layers. Consequently, the elasticity modulus perpendicular to the cover layers has to be sufficiently large, whereas parallel to the cover layers even a very small elasticity module is not detrimental. The characteristic of the core layer can hence be either anisotropic or isotropic. Ultra light core layer structures that have proven successful in practice, are, for example, honeycomb structures made of light metal alloys or resin-impregnated fiber-reinforced paper (anisotropic) as well as rigid expanded foams (isotropic).
In addition, DE-A-197 57 098 discloses a panel connected with a frame, with the frame receiving the panel and providing a connection with other components. Depending on the specific implementation, the frame can also be formed by a mounting wall in which the panel is to be integrated. The connection between the panel and the frame is typically designed as an elastic connection which exerts on the oscillating panel either no resistance at all or only a small resistance. Also known are rigid connections wherein the panels are fixedly connected to the frame.
The panels are driven by drivers whichxe2x80x94as illustrated in DE-A-197 57 097xe2x80x94are either located on the respective panel or integrated with the panel.
It is also known to install drivers in form of, for example, electrodynamic shakers or piezoelectric bending oscillator disks primarily in the center or in close proximity to an outer edge, although an analysis of individual undisturbed oscillation modes of rectangular panels may also suggest other suitable locations. It has proven difficult to optimize the excitation position when taking into account the driver feedback, the large number of, in particular, low-frequency modes and the acoustic contribution of each of oscillations mode at each respective modal frequency. A possible solution may be based on modeling the excitation position by a finite element method in combination with a numerical solution of the acoustic field equations, and with a stochastic variation of the boundary conditions and the exact positions over a range of realistic tolerances. Another solution would be to test in practice random driver positions on finished panel loudspeakers. Both approaches are very complex.
It is therefore an object of the invention to define positioning areas for drivers relative to the surface of the panel, wherein the drivers can be easily and efficiently placed in these positioning areas.
According to an aspect of the invention the positioning area extends between an edge area, which is immediately adjacent to the edges of the panel in the direction of the center of gravity of the panel, and a center-of-gravity area, which extends around the center of gravity of the panels, then obtainable oscillation modes are efficiently utilized while at the same time eliminating harmful local impedances.
If according to claim 2 the panel is fixedly clamped in the frame, wherein the width B of the edge area should correspond to at least 5% of the diagonal of the panel in order to reduce local impedances. In particular, local impedances are reduced for a fixedly mounted panel if the width B of the edge area is approximately 10% of the diagonal of the panel. To increase the efficiency of the oscillating modes, the center-of-gravity area should have a diameter D of at least 20% of the diagonal of the panel. Smaller values of the diameter super-proportionally exclude oscillating modes for driving the panel.
According to another embodiment of the invention, the panel is connected to the frame by yieldable elements, wherein the center-of-gravity area should be cross-shaped, because the areas which are directly adjacent to the lines bisecting the centers of the edges and the center of gravity of the panel have proven to be inadequate for positioning the drivers.
According to yet another embodiment of the invention, the center-of-gravity area is cross-shaped, so that four positioning areas are obtained. To reduce the effect from the edges of the panel on these positioning areas, these areas should include a reduction in those regions where two respective edges of the panel form a corner.
To completely eliminate the influence of the corners of the panel, the reductions should have a triangular shape, wherein two sides of each triangular-shaped reduction are formed by the inner edges of the edge area and the remaining edges of the triangular reductions are located on a closed continuous line that connects the centers M of the edges.
According to yet another embodiment of the invention the shape of the panel is elongated rather than square, wherein the width of the edges of the panel that have a different length should also be different.
The width B1 of the edge area which extends along the long edges of the panel, is a greater than the width B2 of the edge area which extends along the short edges of the panel.
The width B1 is at least 10% and B2 is at least 5% of the diagonal of the panel.
In order to eliminate the aforedescribed disadvantages and/or to obtain a relatively large area for positioning the drivers, it is not necessary that the two two-dimensional areas that form the cross-shaped center-of-gravity area have the same width.
Instead, it is sufficient if the two two-dimensional areas that extend parallel to the long edges of the panel have a width 3.1 that is larger/equal to 2.5% and the two-dimensional areas that extend parallel to the short edges of the panel have a width 3.2 that is larger/equal to 17% of the diagonal of the panel.
An optimal positioning area for the drivers is provided if the drivers have a distance A1 to the center line Mxe2x80x2 that extends parallel to the long edges of the panel and a distance A2 to the center line Mxe2x80x3 that extends parallel to the short edges of the panel.
The distance A1 should be approximately 7% and A2 approximately 14% of the diagonal of the panel.